I don't actually own one waiting for NA release but from many online hype reviews via YouTube, GBAtemp and even professional review sites.
I've gathered that the New 3DS's buttons in general have a better feel to them then the old 3DS, they are firstly slightly bigger in the case of the face buttons and D-Pad on the XL models and have a more precise clickity feel to them however no one can speak for how durable they are compared to the old 3DS's face button and D-Pad.
As for the shoulder buttons, they are said to follow same trend of the new more precise clickity feel of the face buttons/D-Pad but they are smaller in size compared to the old 3DS's shoulder buttons mainly to make room and proper spacing for the additional ZR/ZL buttons and same goes for durability only time will tell.
I've not seen or read anything that has been changed about the left analog stick so it's very likely it's the same as the old 3DS's left analog stick but I really would have liked it if Nintendo were to use a rubber coating on the top part of the left analog stick for the New 3DS and maybe made it slightly bigger like the face buttons and D-Pad on the New 3DS (we're talking 0.X mm here but it does make a difference IMHO).